The present invention relates to ultrasonic level detectors, and more particularly to an ultrasonic level detector having improved signal processing features.
Ultrasonic level detectors are used to determine the level of liquids or dry bulk materials within various types of containers such as storage tanks, pipes, etc. Ultrasonic level detectors of this type generally determine the level of the liquid or material by using a transducer to generate a burst of ultrasonic energy towards the surface to be detected. The energy burst is reflected by the surface and the reflected echo is detected by the transducer. The level of the surface within the tank is determined based upon the amount of time required for the energy burst to travel from the transducer to the surface and back.
While the operation of such a detector is simple in concept, various practical problems arise in the implementation of such devices. One such problem involves the reception by the transducer of multiple echoes of the burst of energy or echoes from the internal structure of the container, such as internal pipes, for example. In this case, the detector might have difficulty distinguishing the echo corresponding to the surface to be located from the other echoes. Also, random electrical noise may be generated in the processing circuitry of the detector by adjacent devices, such as motors for example. These spurious electrical signals may also cause the detector to have difficulty in properly detecting the echo.
One attempt to remedy this problem is described in U.S. Pat. No. 4,901,245 to Olson, et al., which involves the use of a "receive window" of a variable size. In order for the Olson system to recognize an echo signal as one that corresponds to the surface being located, the echo signal must be detected within the receive window. All electrical signals received outside the receive window are thus considered by the Olson system to be spurious signals that do not properly represent the level of the surface.
The Olson system, which determines whether the echo signal occurs within the receive window through the use of a computer program, is primarily a software-based system and does not appear to provide redundancy in checking the validity of echo signals. Moreover, the Olson system does not appear to provide any manner of attenuating electrical signals that are received outside the receive window.
Moreover, the Olson system is not a through-air system, but is instead a relatively complicated, nonintrusive through-liquid system. Such a nonintrusive, through-liquid system must radiate bursts of sonic energy through both the wall of the container and the liquid in the container. The walls of the container may be of various materials, such as steel or plastic for example, and may have various thicknesses. The amount by which the sonic energy is attenuated depends upon both the wall material and thickness. The liquid within the container may also contribute a varying degree of attenuation since different liquids may have different attenuation coefficients.
Consequently, the nonintrusive, through-liquid Olson system includes complicated means for compensating for various wall materials and thicknesses and for different liquids stored in the tank. For example, the Olson system includes a calibration routine in which the transducer is excited with various frequencies to generate echo signals the magnitudes of which are stored and analyzed. The use of such compensation schemes is not required in intrusive, through-air systems since the ultrasonic energy bursts are not required to travel through the container wall and the liquid in the container.
U.S. Pat. No. 4,487,065 to Carlin, et al. discloses an intrusive, through-air ultrasonic level detection system that utilizes receive windows. However, the Carlin system requires a series a periodically spaced reflective disks in the direct path between the transducer and the liquid surface being detected. Consequently, the Carlin system has a relatively complex windowing scheme which requires the utilization of a separate receive window for each of the reference disks. The use of such a complicated detection scheme is believed to be unduly complicated and costly to implement.
Another problem in the operation of level detectors is that the strength of the reflected echoes, and the corresponding magnitude of the electrical signals generated therefrom, may vary considerably. This may be caused, for example, by the fact the level of the surface being detected may vary, so that the distance travelled by the burst of energy may also vary. Because the energy burst is attenuated by an amount corresponding to the distance it travels, the resultant electrical signals generated by the transducer will have varying magnitudes.
Many different approaches to remedy this problem have been proposed. Most of these approaches involve the use of some type of automatic gain control, wherein the gain of an amplifier in a level detector is varied depending upon the amplitude of the electrical signal received by the detector. While some of these approaches may be satisfactory, they offer limited operational advantages and flexibility of operation.
For example, U.S. Pat. No. 3,985,030 to Charlton discloses an ultrasonic detector that generates a transducer excitation signal having a variable number of pulses. The Charlton detector also has a variable gain device for determining the gain of a detector in accordance with the distance that bursts of energy must travel. However, the Charlton detector does not change both the gain and the number of excitation signals based upon the magnitude of an echo signal.